The present invention relates generally to a light emitting diode (LED), and more specifically to a flip-chip light emitting diode with resonant microcavity for enhanced light extraction efficiency.
A light emitting diode (LED) is a semiconductor device that generates light from electrical excitation. It would be advantageous for many applications to improve the efficiency (radiant power out divided by electrical power in) of LEDs. One of the primary limitations to nitride LED performance is extracting light from the semiconductor. The problem is that semiconductors generally have large refractive indices (n). The large refractive index causes most of the light to be internally reflected at the front surface due to total internal reflection, with the net result that only around 5% of the light can escape in LEDs using typical III-V semiconductor alloys.
One method for improving the light extraction from an LED is to use a microcavity LED, which is also known as a resonant-cavity LED. These LEDs place the light emission layer inside a Fabry-Perot cavity having dimensions selected to create a resonance tuned to the emission wavelength of the LED. Use of a resonant microcavity increases the light emission into the vertical modes (which are more easily extracted) by reducing the total number of optical modes in the device. The rest of the light is emitted into waveguided modes that travel lateral to the surface of the LED. Representative patents describing resonant-cavity LEDs include U.S. Pat. No. 6,438,149 to Tayebati, et al; U.S. Pat. No. 5,068,868 to Deppe, et al.; U.S. Pat. No. 5,226,053 to Cho, et al.; and U.S. Pat. No. 5,362,977 to Hunt, et al.
Using a thin microcavity minimizes losses due to waveguided light; the thickness need be only a few wavelengths. Extraction efficiencies on the order of 30% to 50% for semiconductors of interest have been predicted. One of the microcavity reflectors (e.g., rear reflector) generally has a very high reflectivity (>95%); while the other microcavity reflector (e.g., front reflector) is partially reflecting, and can have a reflectivity as low as 25%. In addition to optimization of the cavity's reflectors, the light extraction efficiency is maximized by placing the light-emitting active layer (i.e., emissive layer) at an anti-node of the cavity. The electric field is a maximum at the anti-node, so the emitted light couples most strongly into the optical mode when placed at the anti-node.
LEDs with optical resonant microcavities typically use distributed Bragg reflectors (DBRs) for the front and rear reflectors. Distributed Bragg reflectors typically consist of a stack of alternating semiconductor materials with different refractive indices and quarter-wavelength thickness. A larger number of layers are required to achieve high reflectivity if the difference in refractive indices is small. A larger number of layers could increase electrical resistance losses due to the large number of interfaces or if some of the layers are difficult to dope. Also, it might be difficult to maintain material quality with a larger number of layers—particularly if the layers do not have the same lattice constant. Distributed Bragg reflectors are particularly difficult to fabricate for alloys of aluminum-indium-gallium-nitride (AlInGaN), because the Al-alloys are difficult to dope (p-type is more difficult than n-type), and because there is a large lattice mismatch between AlGaN and GaN compounds.
LEDs using InGaN alloys for the emissive layer emit in the violet to the green wavelengths, which are very useful for a variety of display, signaling, and illumination applications. LEDs using AlInGaN alloys for the emissive layer emit in ultraviolet wavelengths (about 240 to 400 nm), which are useful for a wide variety of applications (e.g., sensors, communication, etc.) When the emissive material is grown on a substrate that is electrically insulating (e.g., sapphire), both p and n-contacts are located on the epitaxial (i.e., front) surface. Consequently, front-surface emitting LEDs (e.g., AlInGaN LEDs) have reduced light extraction due to the parasitic absorption by the metal contacts, bond pads, and wire bonds to the submount or device package.
Flip-chip configuration refers to flipping the chip upside down so that light is now emitted through a transparent growth substrate (e.g., sapphire), as illustrated schematically in FIGS. 1 and 2. A flip-chip configuration has a number of advantages over the standard configuration where light is emitted from the GaN surface (i.e., “front-surface emission” configuration). Firstly, it has superior thermal dissipation compared to the conventional front-surface emission design because heat is no longer conducted through the sapphire growth substrate (sapphire has a much lower thermal conductivity compared to the ceramic or silicon substrates typically used for the submount), but, rather, is conducted directly to the submount through high thermal conductivity metallic contact pads and interconnects. Secondly, the flip-chip geometry has lower series resistance compared to a front-emission geometry because a thicker metal p-contact pad can be used as a current spreader (front-surface emission requires an optically-transparent p-contact pad to transmit the light, which requires a very thin metal layer). The lower series resistance also means that the LED can be made larger for higher total flux. Another advantage of the flip-chip configuration is the fact that the rear metal p-contact pad (depending on the choice of metal or metal alloy) can also function as a reflector (i.e., mirror), which increases the total light output by reflecting light initially emitted from the light emitting layer (e.g., MQW in FIGS. 1 and 2) towards the rear p-contact back towards the sapphire substrate, where it passes through and escapes.
Conventional flip-chip LED's, such as those illustrated in FIGS. 1 and 2, do not utilize resonant microcavities to enhance light extraction efficiency. Representative patents describing flip-chip LEDs include U.S. Pat. No. 6,455,878 to Bhat, et al.; and 6,483,196 to Wojnarowski, et al.
What is needed, therefore, is an improved LED design that combines the beneficial features of a flip-chip configuration with the enhanced light extraction efficiency of a resonant optical microcavity.
Against this background, the present invention was developed.